canberrabirds

Bold eagles take flight

To: "Canberra Birds" <>
Subject: Bold eagles take flight
From: "Tony Lawson" <>
Date: Wed, 13 Jul 2011 22:08:15 +1000

If you spot two enormous birds locked together in what appears to be a soaring, tumbling battle for dominance of the sky, then look again! You're most probably watching a pair of wedge-tailed eagles re-igniting their passion prior to breeding.

Wedge-tail eagles are among the world's largest eagles with a wingspan of up to 2.5 metres. They are relatively common all over the Australian mainland, in Tasmania and southern New Guinea, except in densely populated areas.

To get off the ground they use an active flapping flight, but once airborne these eagles use thermal updrafts to carry them effortlessly aloft and gracefully spiral upwards to great heights.

Australia has three species of eagle, the Little Eagle (Hieraetus morphnoides), the White-bellied Sea Eagle (Haliaeetus leucogaster) and the largest, the Wedge-tailed Eagle (Aquila audax - literally 'bold eagle'). Wedge-tails prefer open woodland, but are found exploiting a range of habitats from arid desert to grasslands, mountainous areas, and even rainforest.

Easily identified by their size (up to 3.2 kg for males and 4.2kg for females), Wedge-tails are known as booted or trousered eagles thanks to the heavy feather 'trousers' covering their legs. They are also characterised by finger-like wing feathers and, of course, a long wedge-shaped tail. Young birds can be distinguished by their light brown plumage and golden highlights; the plumage of older birds tends to darken to near black at sexual maturity between five and seven years.

Fiery courtship

According to Dr Penny Olsen, a research scientist and raptor expert from the ANU, Wedge-tails appear to be monogamous, mating for life. "They certainly form long-term pair bonds," she says, "although if one bird is killed the survivor will take a new mate."

The time of breeding depends on temperature to some extent. In Tasmania, Wedge-tails breed later than their northern counterparts, but all breed between June and October.

"Breeding season is the best time to see Wedge-tails because of their highly visible soaring and diving displays," says Olsen. Males may also mark their territorial boundaries by flying in a series of showy descending loops, or 'pothooks'.

In another dramatic manoeuvre known as talon grappling, two birds lock talons with one being forced to flip over and fly upside-down, which although spectacular can be fatal if one bird refuses to let go. Olsen says the research community is divided on whether this is an aggressive display aimed at warning off potential interlopers, or just two birds showing off as part of their courtship ritual.

Position, position

Breeding pairs establish territories which vary in size depending on the abundance of prey. These territories are used throughout the year, and within them the eagles may build several nests, some of which are used as feeding platforms.

Seeking a clear view over a wide area, they generally build their nest in the largest tree available. And it needs to be big: while a nest can start as small as one metre in diameter and just 75cm deep, as sticks are added over successive years, the structures can grow up to four metres deep and two metres in diameter, and weigh in at around 400kg.

At laying time a shallow depression in the centre of the nest is lined with fresh leaves, and between one and three eggs (usually two) are laid at two-four day intervals. The male and female take turns to incubate the clutch for approximately 45-47 days, and then the eggs hatch one at a time.

Fact file:

When: June to October

Where: Wedge-tailed eagles are found throughout the Australian mainland and Tasmania.

Nestling skulduggery

During the first five weeks adults place food directly into the chicks' mouths, but from then onwards the chicks can recognise and pick up food from the nest floor. The parents also protect their young from predators and the elements; if threatened, larger chicks lie flat in the nest, but will put up a spirited defence if necessary.

However, the greatest threat to a chick's survival is not predation, but its oldest sibling: although survival rates vary considerably depending on local conditions usually only the strongest chick survives. In a good year, a pair of chicks may be fledged, but when times are tough, the oldest chick will kill and eat its nestmates.

Once fledged, the young eagles remain with their parents for up to three months before gradually dispersing up to several hundred kilometres away. If they survive their first year (and two-thirds don't), wedge-tailed eagles can live for at least 40 years in the wild and more than 60 in captivity.

Past misunderstandings

The birds' large size means that they can take a whole range of prey from small lizards to ground-foraging birds, possums, wallabies and kangaroos. Lacking the speed of other raptors, Wedge-tails are designed for intercepting prey on the ground, and will even cooperate with other birds to hunt large prey; Olsen says several eagles will cooperate to bring down a large kangaroo.

On the whole, the birds have benefited from European arrival in Australia - woodlands have been opened up for pasture and many introduced species, including cats, foxes, and particularly rabbits, are the perfect sized prey for a hungry eagle.

However, while they actively hunt live prey during the breeding season, Wedge-tails are enthusiastic carrion eaters, and have frequently been observed eating recently deceased lambs. For many years angry farmers believed the birds were picking off their young stock, and more than 30,000 birds were killed in the 1970s for what research has now revealed a major misunderstanding.

These days it is recognised that few Wedge-tails actively hunt lambs and calves; ironically most are actually doing the farmers a service by rapidly consuming any dead animals and so minimising the chance for flies to breed. There are still occasional 'misunderstandings' between farmers and eagles, but they are now protected in all states.

Rabbits and Windfarms

While they lack natural predators a couple of new threats to Wedge-tail survival have emerged in the past decade or so: rabbit Calici-virus and the growth of windfarms in eagle hunting grounds.

Like other animals that relied heavily on rabbits Wedge-tailed Eagles experienced some local population declines in the aftermath of the Calici-virus outbreak. But although the virtual disappearance of their favourite prey caused serious short-term consequences for some rabbit-dependent populations, Wedge-tails are such generalist predators that they simply switched to eating other things and overall numbers have not suffered.

Windfarms have also caused concern among environmentalists for years, who fear that birds may blunder into the slashing turbines. Dr Olsen says that occasionally Wedge-tails have met their end that way.

"Normally, they know where the turbines are and avoid them," she says. "However accidents tend to happen - including two recent casualties in Tasmania--when it is foggy or there are high winds that interfere with the birds' normal navigational abilities."

Most governments are aware of the problem and try to minimise the impact of any new developments on the local bird life. "New turbines in development, in which the fan sits horizontally, might also help reduce their impact," says Olsen.

Credits

Special thanks to Dr Penny Olsen and Peter Merritt.

http://www.abc.net.au/science/articles/2006/07/06/2041996.htm 

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